The chairman of the U.S. Senate's commerce committee wants Apple to lift the veil on its once-secret slowdown of older iPhones.

Sen. John Thune, a South Dakota Republican, issued his request for more information in a letter sent Tuesday to Apple CEO Tim Cook.

Among other things, Thune wants to know if Apple made any effort to notify its customers that its recent software updates would make several types of iPhones released before 2017 slow down when their batteries weakened.

Thune also asked Apple whether it considered replacing the batteries for free, instead of the current discounted charge of $29 that it began offering in late December after acknowledging how its updates affected older iPhones. He also inquired whether Apple plans to throttle aging iPhones in the future and whether it plans to let consumers know what it's doing.

Thune demanded a response by Jan. 23. Apple declined to comment.

The inquiry represents the latest backlash against Apple's decision to slow down older iPhones, a move that the company says is designed to prevent the devices from abruptly shutting down when older batteries are running low or operating in cold weather.

Although technology analysts have mostly defended Apple's strategy as a way to make older iPhones last longer, some regulators and many consumers believe the company has been manipulating the way aging devices work to spur sales of its latest -- and more expensive -- models.

French authorities are investigating whether Apple is engaging in a pattern of "planned obsolescence" in violation of France law and a variety of consumer lawsuits have been filed in the U.S.

Apple has responded with an apology and reduced the price to replace batteries on older iPhones by $50.